PS plates comprising a positive type sensitizing agent containing a diazo compound and a phenolic resin as main components or a negative type sensitizing agent containing an acrylic monomer or prepolymer as main component have heretofore been put into practical use as lithographic offset printing plate precursors. However, since all these printing plate precursors have a low sensitivity, these printing plate precursors are exposed to light through a film original on which an image had been previously recorded brought into close contact therewith to form printing plates. On the other hand, the progress of computer image processing techniques and large capacity data storage and communication techniques have recently enabled a continuous computer based operation including original input, correction, editing, layout and paging. With this computer operation, an electronic editing system capable of instantly outputting data to terminal plotters via a high speed communications network or satellite communications network has been put into practical use. In particular, such an electronic editing system is in great demand in the field of newspaper printing requiring instantaneity. Furthermore, in the field of printing wherein a printing plate is reproduced as necessary based on an original stored in the form of film original, a tendency will be growing that originals are stored as digital data in very large capacity recording media such as optical disc which will be developed.
However, little or no direct type printing plate precursors designed to directly receive data from the output of terminal plotters to form a printing plate have been put into practical use. Even in stations where an electronic editing system is operated, data is outputted to a silver salt system photographic film. PS plates are then exposed to light through the silver salt system photographic film brought into contact therewith to form printing plates. One of the reasons for the above described conditions is that it is difficult to provide a direct type printing plate precursor having sufficient sensitivity to form a printing plate within a practical period of time by a light source in the output plotter (e.g., He-Ne laser, semiconductor laser).
An electrophotographic photoreceptor can be a light-sensitive material having a light sensitivity high enough to provide a direct type printing plate. Many electrophotographic printing plate precursors of the type wherein a photoconductive layer in the nonimage portion is removed after the formation of toner image have already been known. Examples of such electrophotographic printing plate precursors include those described in JP-B-37-17162, JP-B-38-6961, JP-B-38-7758, JP-B-41-2426 and JP-B-46-39405 (the term "JP-B" as used herein means an "examined Japanese patent publication"), and JP-A-50-19509, JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-56-107246, JP-A-57-147656 and JP-A-57-161863 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
JP-A-56-107246 discloses an electrophotographic printing plate precursor prepared by a process which comprises adding an oxazole compound as an organic photoconductive compound to a binder resin soluble in an alkali or aqueous alcohol solution, adding a sensitizing dye to the mixture, and then coating the material on an aluminum plate as a photo-conductive layer. JP-A-56-146145 discloses an electrophotographic printing plate precursor comprising an oxadiazole compound as an organic photoconductive compound, a condensed polycyclic quinone pigment as a charge generating agent and an alkali-soluble carboxyl group-containing polymer. JP-A-62-54266 discloses an electrophotographic printing plate precursor comprising a hydrazone compound as an organic photoconductive compound and a pigment or dye such as phthalocyanine pigment or quinacridone pigment and an aqueous or alcohol solvent-soluble binder resin as charge generating agents.
Thus, in order to use an electrophotographic photoreceptor as a printing plate, it is normally necessary to remove the nonimage portion with an alkaline etching solution to allow the hydrophilic surface to be exposed. Therefore, as a binder resin there is often used a binder resin which is dissolved in or swells in an alkaline solvent to undergo elimination. However, as compared to polycarbonate resins which have been widely used as binder resins for electrophotographic photoreceptors, such a resin which is dissolved in or swells in an alkaline solvent is normally poor in the compatibility with any organic photoconductive compound such as oxazole, hydrazone, oxadiazole and pyrazoline. This causes a problem that when such an organic photoconductive compound is dissolved and incorporated in a printing plate, it is separated and deposited thereon with time. Furthermore, since such an organic photoconductive compound has a poor solubility in an etching solution and thus lacks elutability by an etching solution in the nonimage portion, stains are observed in the white background of the resulting printed matter.
Moreover, an electrophotographic printing plate precursor comprising such an organic photographic compound normally has poor chargeability and capability of retaining electric charge in dark places as set forth in Examples of the present invention. Therefore, the potential contrast (potential difference) between the image portion and the nonimage portion is lowered, causing some troubles in toner development or some restrictions in the time between charging and development. This results in the formation of an electrophotographic printing plate precursor with a poor practicality. Thus, an electrophotographic printing plate precursor comprising an organic photoconducting compound soluble in a binder resin has many disadvantages.
On the other hand, an electrophotographic printing plate precursor free of such an organic photoconductive compound soluble in a binder resin is known. Such an electrophotographic printing plate precursor comprises a photoconductive layer having an organic photoconductive pigment such as a phthalocyanine pigment dispersed in a binder resin soluble in an alkaline or alcohol aqueous solution. For example, an electrophotographic printing plate precursor comprising an aluminum plate having thereon a photoconductive layer having a phthalocyanine pigment dispersed in a phenolic resin is disclosed in JP-A-55-105254 and JP-A-55-161250. However, such an electrophotographic printing plate precursor is disadvantageous in that it lacks sensitivity. Thus, it is known that an electrophotographic photoreceptor comprising a phthalocyanine pigment dispersed in a binder resin and free of an organic photoconductive compound such as a hydrazone compound or oxazole compound exhibits an induction effect which causes a drop in the sensitivity (Weigl, "Current Problems in Electrophotography"- P278, Walter de Gruyter, 1972). It is also known that the incorporation of an electrophilic compound such as tetranitrofluorenone or trinitrofluorenone in such a photoreceptor enables a reduction in the induction effect and an improvement in sensitivity ("Denshi Shashin Gakkaishi" 60, 116, 20, 1982). However, such an electrophilic compound is toxic and can hardly be put into practical use.
As mentioned above, it has been desired to provide an elecrophotographic printing plate precursor with a high sensitivity, no aging deterioration and an excellent elutability by an etching solution.